310 CRETACEOUS PELECYPODA 



17. Barhiitia, Gray, 1810. Shell oblong, moderately tumid, radiately striated 

 and covered witli a hairy epidermis; ligameatal hinge area narrow; hinge line 

 sliHitly curved, with the ends somewhat dilated and provided with numerous 

 oblique teeth, increasing in size on both sides ; sometimes the median teeth become 

 obsolete; type, B. harhala, Linn. 



The forms of Barhalla are somewhat variable. The typical ones have the 

 ventral side provided with only a very small gape in the anterior region. Others 

 have the ventral margins more or less insinuated, the gape being enlarged, and the 

 anterior part of the shell becomes narrow and produced. To the latter type belongs 

 the recent A. velata. Both forms pass insensibly one into the other through such 

 species as Bar. fiisca, Brug., which is extremely variable, almost more so than 

 Bti)'. velata, but in all species the ends of the hinge line are peculiarly flattened. 



17a. CucullcBarca, Con., 18G5(?), (Am. Journ. Conch., I, p. 11). This has 

 been projiosed for a few eocene species of North America, Bijssoarca lima. Con., 

 (Journ. Acad. N. Sc, Phil., 2nd ser., i, p. 125, pi. 13, fig. 23,) being considered as the 

 type. This, as well as another species, B. Misslssippensh of the same author, do 

 not appear to me to exhibit any essential differences from Barhatia, jmrticularly of 

 the B. velata type. Both are somewhat sinuated in front, radiately striated, and 

 the hinge has oblique teeth, increasing in size towards the ends of the hinge line. 

 Whether the genus possesses any other peculiarities, distinct from those of Barhatia, 

 cannot be concluded from the description of the two species, for I have not come 

 across Mr. Conrad's detailed characteristic of the genus. 



176. Acar, Gray, 1851 (?). This should include the somewhat more tumid, 

 cancellated, and along the posterior slope carinated species; they are mostly of 

 small size, as Bar. dicaricata and piisllla of Sowcrby; it forms a convenient 

 section of Barbatia. 



Vic. For Area alternata of Reeve, Gray proposed the name Calliarca 

 (or Culloarca), the hinder slope being strongly carinated and both ends strongly 

 ribbed. It is scarcely necessary to separate this into a distinct sub-genus, for there 

 are numerous intermediate cancellated forms which connect Barhatia with true Area 

 and Litharea. 



Yld. Striarca has been proposed by Mr. Conrad for the miocenc Area 

 centenaria. Say. The shell is oblong, with a depression running from the beak 

 to the ventral margin, which is somewhat insinuated, no doubt corresjionding to 

 a small gape ; the surface is radiately striated ; the ligameutal area and hinge-teeth 

 are quite similar to those of Barhatia, from which the form should not, I l^elieve, 

 be generically separated Cvide Conrad's mioc. shi-lls, 1838, p. 55, pi. 29, fig. 1). 



17<?. Litharea, Gray, 1810. This has been proposed for the peculiar recent 

 Area lithodonms, Sow., a shell which is very peculiarly wedge-shaped, elongated, 

 the anterior portion being longer, narrowly rounded in front ; the posterior is shorter 

 and very obliquely truncate ; the ligameutal area is of moderate width ; the hinge 

 line straight in front and somewhat bent down posteriorly. The sub-genus seems 

 sufficiently distinct from other allied forms. 



